1. Field of the Invention
The present invention relates to a Mode S transponder transmission signal decoder and a Mode S transponder transmission signal decoding method for decoding a Mode S reply signal to a Mode S interrogation signal transmitted to a transponder mounted on an aircraft, and a decoding Mode S squitter signal (including a Mode S short squitter signal and a Mode S extended squitter signal) transmitted by the transponder.
2. Description of the Related Art
An aircraft surveillance radar for use in air traffic control is broadly divided into a primary surveillance radar (PSR) and a secondary surveillance radar (SSR).
The above-described PSR emits radio waves from the ground, and receives and processes reflected waves thereof, thereby acquiring positional information of an aircraft.
Meanwhile, the SSR transmits an interrogation signal from the ground, receives a reply signal thereto from a transponder, thereby acquiring a variety of information regarding the aircraft.
Note that modes of the SSR are classified into a Mode A, a Mode C, and a Mode S depending on types of the information to be acquired, in which the Mode A is one for acquiring identification information of the aircraft, the Mode C is one for acquiring altitude information, and the Mode S is one for acquiring track information, speed information, and the like in addition to the above-described information (refer to HASHIDA Yoshio, OOTOMO Hisashi, and KUJI Yoshinori, “Secondary Surveillance Radar for Air Traffic Control—SSR Mode S”, Toshiba Review, Vol. 59, No. 2 (2004), pp. 58-61).
Moreover, a transponder for the Mode S has a function to transmit a Mode S short squitter signal, which has a signal format similar to that of a Mode S reply signal and is composed of a 24-bit address, and to transmit a Mode S extended squitter signal, which has also the signal format similar to that of the Mode S reply signal and represents a position, a speed, and the like of the aircraft itself. Note that, in the following description, the above-described Mode S short squitter signal and Mode S extended squitter signal are collectively referred to as a “Mode S squitter signal” as appropriate.
The above-described Mode S squitter signal is automatically transmitted from the Mode S transponder at a fixed interval, and is receivable not only at a ground station but also at the aircraft. Therefore, the Mode S squitter signal can be used for automatic dependent surveillance broad assistance (ADS-B).
In the case of decoding the above-described SSR Mode S reply signal and Mode S squitter signal, a threshold value has been set as shown in FIG. 1A, and portions, where outputs are larger than the threshold value, and pulse widths are within a predetermined range as shown in FIG. 1B, have been recognized as such signals.
Note that FIG. 1B shows the case where a width of a pulse W is within the predetermined range, a width of a pulse X does not meet a lower limit value of the predetermined range, and a width of a pulse Y exceeds an upper limit value of the predetermined range. Specifically, only the pulse W is recognized as a correct signal.
However, as shown in FIG. 2A, a direct wave 104 of the Mode S reply signal transmitted from an aircraft 101 to a ground station 102 in response to a Mode S interrogation signal 103 from the ground station 102 is sometimes subjected to interference from a reflected wave 105 of the same Mode S reply signal. Moreover, this phenomenon sometimes occurs also in the case of receiving the Mode S squitter signal.
Note that, in the following description, the above-described Mode S reply signal and Mode S squitter signal are collectively referred to as a “Mode S transponder transmission signal” as appropriate.
The interference as described above by the reflective wave is caused by a phase difference between access routes, and a power level P of the Mode S transponder transmission signal received by the ground station 102 is represented by the following expression.P=αsin ωt+B sin(ω(t+d))  (1)
Note that, in the above-described Expression (1), P is the receiving power level, α is an attenuation of the direct wave, B is an attenuation of the reflected wave, and d is a time difference of arrival.
Moreover, when there is a plurality of aircrafts within a surveillance range of the ground station 102 as shown in FIG. 2B, the Mode S transponder transmission signal 104 from the aircraft 101 and a Mode S transponder transmission signal (asynchronous signal) 107 from an aircraft 106 sometimes interfere with each other.
Specifically, if it is assumed that a pulse shown in FIG. 3A is the direct wave 104, and that a pulse shown in FIG. 3B is the reflected wave 105 or the asynchronous signal 107, an associated wave as shown in FIG. 3C is formed when phases of these two pulses are the same. A pulse of the associated wave is misidentified so as to exceed the upper limit value of the above-described predetermined range, and as a result, the pulse concerned is not recognized as the correct signal. Hence, it becomes impossible to correctly decode the Mode S transponder transmission signal.
Meanwhile, when the phases of the two pulses are different from each other, an associated wave as shown in FIG. 3D is formed. A pulse of the associated wave is misidentified so as not to meet the lower limit value of the above-described predetermined range, and as a result, the pulse concerned is not recognized as the correct signal. Hence, it becomes impossible to correctly decode the Mode S transponder transmission signal as in the case shown in FIG. 3C.